MongoDB Custom Increment primary key

Private final static String Column_seq = "SEQ";
    Private final static String column_category = "CATEGORY";

    Private Mongo Mongo;
    Private String dbName;
    Private String collname;

    public void Setmongo (Mongo Mongo) {
        This.mongo = Mongo;
    }

    public void Setdbname (String dbName) {
        this.dbname = dbName;
    }

    public void Setcollname (String collname) {
        this.collname = collname;
    }

    @Override public
    Long Nextseq (String category) {
        if (category = = NULL | | ". Equals (category)) {
            throw new IllegalArgumentException (" category can ' t be null or empty ");
        }
        DBObject update = new Basicdbobject ("$inc", New Basicdbobject (Column_seq, 1L));
        DBObject query = new Basicdbobject (column_category, CATEGORY);
        Return (Long) Mongo.getdb (dbName). GetCollection (Collname). findandmodify (query, NULL, NULL, FALSE, update, True,
                true). Get (COLUMN_SEQ);
    }